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A Deeper Understanding on the Motion Behaviors of Wire during Ultrasonic Wedge-Wedge Bonding Process

Keywords: wedge-wedge bonding, relative motion, thin layer coating

Two interfaces exist within the ultrasonic wire bonding domain - the wire/tool interface and
the wire/substrate interface. The motion behaviors at the wire/tool interface, which could
make the bonding mechanism even more complex, have been rarely investigated. In spite of
many arguments on the occurrence of a relative motion, the relative motion between the wire
and the bonding tool for thick wire bonding has been detected via high speed videos (Long et
al., 2015). Nevertheless, the motion behaviors at the wire/tool interface are assumed to be
more complex than a simple relative motion between two rigid bodies as the aluminum wire is
much softer than the bonding tool made of tungsten carbide. Even though some scratches are
able to be observed on the surface of the wire after bonding, very limited information can be
revealed due to the asymmetric plastic deformation. In the present study, a thin oxide layer
was coated onto the surface of the aluminum wire and the changes of the layer were used to
analyze the motion behaviors. To test the influence of the layer on the bond quality, an
aluminum oxide layer with a thickness of 200 nm was first deposited on the wire. A couple of
bonds with the coated and uncoated wires were then made under the same process parameters
and shear tests were conducted. The result showed an insignificant difference between the
shear forces of the two different kinds of bonds, which means the influence of the layer on the
bond quality is ignorable. Then different oxide layers (Al2O3 and ZrO2) with smaller
thicknesses were coated. The coated wires were bonded for 20 ms, 30 ms, 40 ms and 50 ms
under the same normal force and ultrasonic power settings. Through energy-dispersive x-ray
spectroscopical (EDXS) observation, it was found that most oxides were removed from the
regions where the wire contacted the fillets of the bonding tool within 30 ms. While after 50
ms, the oxide layer on other regions was either removed or reduced. Many oxides are found to
be transported to the outer regions which are close to the fillets contact region. In addition,
after hundreds of usage cycles, the bonding tool was also analyzed by EDXS and the result
showed oxide accumulations on the fillets part as well as on the other contact perimeter
regions. These results indicate that complex relative motion behaviors between the wire and
the tool existed. Both the continuous plastic deformation and the ultrasonic vibration induced
relative motions. The impact of the continuous plastic deformation dominated the relative
motion in the first ~20 ms while the relative motion in the latter stage was dominated by the
vibration. The relative motion amplitude at the fillets contact region is higher than that at the
other contact perimeter region and the amplitude at the central area is the lowest. The wedgewedge
bonding has a very high self-cleaning efficiency both at the wire/tool interface and the
wire/substrate interface.